The Heroes Unite
by JaceHerondale10
Summary: Percy Jackson's world collides with the magical world of the Mortal Instruments in an epic battle to save demigods and the Clave from going to war, but there is another enemy, one they can't defeat without each other. Please note that the Percy Jackson series belongs to Rick Riordan and the Mortal Instruments to Cassandra Clare. I only own the fictitious plot within this crossover!
1. Prologue

**The Heroes Unite**

Prologue: July 12th, 2010

Jace Herondale was jogging through New York when his cell phone rang. He slowed down while simultaneously pulling it out of his pocket, but declined the call when the caller ID said "Unknown Number". He had no idea that his friends were in trouble.

Meanwhile at the Institute, Clary Morgenstern was fuming. "He said he'd be back by 7, Isabelle. IT'S PAST 7:00!"

Isabelle Lightwood just laughed. "Y'know, Clary, he's a Shadowhunter. We _all _are, in case you haven't noticed," Isabelle said. She, on the other hand, was annoyed for a different reason…

Simon Lewis was just coming back from Alicante, the Shadowhunter city, when Jace called him. _Great,_ Simon thought,_ what does Angel Boy need now?_ Simon was a good-natured person, but even Jace Herondale got under his skin. Nevertheless, he resolved to answer the phone, and was immediately torrented by a string of angelic anger.

"DON'T YOU EVER ANSWER YOUR PHONE, ACCURSED MUNDANE!? OR DO YOU THINK THAT YOUR ASCENSION MAKES YOUR MUNDANE ACTIONS RELEVANT?!" Jace screamed. "I don't know what you're playing at, Simon, but I need you to get to the Institute _immediately!"_ He then hung up the phone on a very confused Simon. Simon stood still, perplexed, but came to his senses quite quickly. He got out his brand-new stele and created a Portal to the Institute in New York, prepared to be yelled at some more. What happened when he stepped in was more shocking.

Percy Jackson was taking a walk on the beach when his phone rang. He looked at it, and smiled when he saw the phone flash two words: Annabeth Chase. He answered, and what his girlfriend said shocked him.

"Percy, I need you to get to Camp Jupiter. Find Reyna, Jason, Frank, and Hazel. I've got Nico, Leo, Piper, and Calypso. Please, Percy, this is important. Do it for me, will you?" Annabeth sounded worried.

"Definitely, but what's going on?" Percy asked. "Is everything alright? Are you hurt? Where are we going?" But Annabeth had already hung up. Sighing, Percy began the long walk to the boat his father, Poseidon, had given him.

Percy wasn't complaining, but why did he need the other demigods? And something wasn't right with Annabeth bringing Calypso along. To top that all off, Nico had been avoiding him. Every time Percy walked into the same room as the son of Hades, he would murmur something about being tired and then disappear into the woods. Percy intended to talk to Nico about that, but then his phone rang again. He looked down and saw the name of an old friend flash on the screen. Smiling, Percy answered the phone.

"Did you miss me, Percy?" Simon Lewis asked, trying to hold back a laugh. "You know what, don't answer that. I just called Annabeth and told her to gather your friends… how do you feel about going to Downtown? We need your help."

Percy was confused. "Who's 'we'? What, are you part of a gang or something now?" He laughed uneasily. "You know what, don't answer that."

Simon was silent for a moment, then said, " How do you feel about going demon hunting with me? We, ah, kind of need your help subduing the Chimera."

Percy groaned. "I'll be there. Tell me when and where."


	2. The Battle Begins

**The Heroes Unite**

Chapter 1: The Battle Begins

Simon Lewis was waiting by Madison Square Garden when he saw Percy Jackson, his old friend from elementary school. Percy grinned as he ran over to the park bench where Simon was sitting.

Simon grinned and said, "Well, well, Mr. Jackson. You're late. I was starting to get worried. But there's no time to talk. Can we take a boat to the Institute? I need you to meet some people."

Percy nodded. "I'll see you at the pier. But… who exactly are we meeting?"

Simon was silent for a moment, then said, "We're going to meet some Nephilim who might be able to help, once they know what we're up against."

Percy thought for a moment. "What exactly are we up against?"

Simon sighed and said, "We'll talk about that once everyone's at the Institute."

Meanwhile, Annabeth Chase, daughter of Athena, had gathered the assembled demigods. In front of her was Jason Grace, son of Jupiter. Jason was a formidable opponent, and an honorable friend. Next to him stood Piper McLean, daughter of Aphrodite. Piper was not your typical romance-based demigoddess. Her half-brothers and half-sisters were absolutely terrified of her, because "she was as deadly as she was beautiful", according to Jason, her boyfriend. On Piper's right, Nico di Angelo, son of Hades, and his half-sister Hazel Levesque, daughter of Pluto. They each had control over their father's two spheres of power. Nico controlled the power of death, and Hazel handled the riches under the earth. Hazel's boyfriend, Frank Zhang, son of Mars, stood to her right. Frank was also technically related to Percy, but that wasn't too important. Reyna Arellano, daughter of Bellona, stood to Jason's left. Bellona, Roman goddess of war, definitely had her warlike nature reflected in her daughter. On Reyna's left stood Leo Valdez, son of Hephaestus. Leo was the jokester of the group, something they all needed. They all stood there, waiting for Annabeth to begin.

Annabeth smiled and said, "You all know why we're here. A worldwide group of demon hunters that call themselves the Clave has attacked one of our bases in Denver, near a piece of their own land. I've managed to access a communiqué between one of their people in New York, and old friend of Percy's. Apparently there's a threat greater than the Clave, a threat that is attempting to get rid of both groups in order to plunge the world into chaos. I don't believe that. Therefore, Chiron has given me the executive authority to track down and capture these people: Isabelle and Alec Lightwood, Jace Herondale, Clarissa Morgenstern, Emma Carstairs, and Julian Blackthorn. You have your orders. Don't fail, demigods."

Jace Herondale was walking down 22nd street when he saw a young boy with a skull ring on his right hand. The boy cried out in pain, so Jace ran to him. The boy was kneeling, cradling his left arm.

Jace looked down on him and said, "What's wrong, little guy? Do you need an ambulance?"

Nico di Angelo set his hand on Jace's knee, grinning evilly as Jace fell to the floor.


	3. The Clave

**The Heroes Unite**

Chapter 2: The Clave

Maryse Lightwood was not having a good day. Max, her youngest child, was at the Paris Institute. Alec, her eldest, was at his boyfriend Magnus' house, helping to clean up some of the mess Jace had made last Thursday. Isabelle, her middle child, was in Alicante, the Shadowhunter city. Robert, her husband, was with Isabelle. So, of course, she didn't have much help at the Institute. At least, she wasn't expecting any.

The Institute doors opened, and Simon Lewis tumbled in, yelling, "Mrs. Lightwood?! Something's happened to Jace, Alec, and Isabelle! Clary can't find them!"

Maryse ran into the foyer, panting slightly. "What's wrong, Simon? You said something about Jace, Alec, and Isabelle. Is Max alright?"

Simon nodded, in shock, as he recounted what he had seen.

_To Whomever It May Concern,_

_Your people have made a dangerous incursion on Olympian territory located in Denver, CO. My people have prepared a ransom exchange of sorts. We have 3 of your own kind: Jace Herondale and his adoptive siblings, Alec and Isabelle Lightwood. We will return them in exchange for a deserting of our territory in Denver. The land you have taken belongs to the gods of Mount Olympus, and your angel cannot save your people. We will be lenient, however. Leave our land, and you may have your warriors back. You have until sunset on July 19__th__. If you refuse our terms, there will be war. You have been warned. –Annabeth Chase_

"So you see, Mrs. Lightwood, we need to get to Alicante _immediately!_ They could be torturing them," Simon said.

Maryse sighed. "Very well, Simon." She turned to leave, but halted. "What about that old friend of yours, Pedro Jorgenson? Isn't he in league with these people?"

Simon laughed. "You mean Percy Jackson? Yea, he's their leader, but I don't see what that's got anything to do with… Wait a minute," Simon said as realization dawned upon him. "Mrs. Lightwood, you're a genius!"

Simon ran out of the Institute, pulling out his phone as he did so. "Percy, come on, pick up," Simon muttered as he ran. Finally, Percy answered.

"Simon? What's up, brother?" Percy asked. "Any problems at your Institute?"

Simon replied, "Remember when you mentioned that chick, Annabeth Chase? She's your girlfriend, right? Well, she's kind of gone and kidnapped Jace, Alec, and Isabelle. Something about territorial incursion in Denver."

Percy drew a sharp breath. "Simon, I need you to meet me at the U.S.S _Intrepid_. I may need to explain a couple stuff."

Simon, obviously confused, asked, "What time? You know, just a rough estimate."

Percy sounded agitated. "How about _ASAP_?"

The phone clicked off. Simon got into his car and drove as fast as he could, careful to stay within speed limit._ Great. First Angel Boy, now this_, Simon thought to himself. He slowed down as he saw the ship. At the bow was a familiar looking face, and the Farsighted rune helped Simon make out the sea-green eyes of Perseus Jackson.


	4. See You at Sunset

**The Heroes Unite**

Chapter 3: See You At Sunset

The day of the ransom delivery had come, and members of the Clave and Downworld were waiting for sunset at an undisclosed rendezvous point. Amongst them were Clary Morgenstern, Simon Lewis, both Nephilim, along with Maia Roberts and Magnus Bane, werewolf and warlock, respectively. Clary was getting impatient. What was it that these demigods wanted from them, and why were Jace, Alec, and Isabelle important in this situation? Among other thoughts, these were at the tip of Clary's mind. As leader of this negotiation team, she was about to call off the sneak attack on these demigods when she saw a young woman walking over the hill, towards them.

"My name is Reyna Arellano, praetor of 12th Legion Fulminata," she said. Reyna looked like a born leader, with a flowing purple cloak and high, arched cheekbones to match her dignified posture. "I assume you have come to retrieve your people? It would be a shame for you to have come all this way just to betray your end of the deal."

Simon frowned. As a gamer, he was familiar with battle strategy, and his sharpened eyesight spotted armed troops approaching, carrying an eagle standard. He knew that the eagle was the symbol of Rome, so it made sense that these were troops from the Roman demigods' side. Mixed in with the standard Roman armor, however, he could pick out Greek equipment as well. Simon's heart sank. They had a similarly planned surprise attack. He briefly registered an _attack_ gesture, and the demigods opened fire. Surprisingly enough, the Clave was ready.

Julian Blackthorn and his _parabatai_, Emma Carstairs, were blurs of deadly force as they cut through enemy lines, seraph blades hissing where they made contact with demigod armor. Their orders were to injure, not kill, so the young Nephilim complied. Meanwhile, the demigods had their own powerful troops.

Nico di Angelo, son of Hades, was enjoying himself as he shadow-traveled through the waves of angelic warriors, distracting them so that Jason Grace, son of Jupiter, could electrocute them. Similar to the Clave, the demigods had received strict instructions to injure, not kill. Naturally, Jason had to be careful with his lightning.

"STOP!" A voice rang out over the mountaintop. It was Magnus. "Can't you see, this is what he wants!" Magnus was clearly distraught, but his voice shook with such ferocity that everyone stopped fighting and looked at him.

"What who wants?" Jason called out. "If you want us to stop fighting, tell us."

Magnus breathed heavily, his breath visible in the mountain air. "A demon. I fear Lilith has returned once more to the mortal world."


	5. Chapter 4

The Heroes Unite

Chapter 4: An Explanation

"Who exactly is this Lilith? What threat does she pose?" Percy asked. As with the other demigods, he understood nothing about demons. Monsters, gods, and giants weren't easy to kill, but he understood them. Demons, on the other hand, were a completely alien species to him.

Magnus looked up at him. "You see, demigod, before gods and monsters, there were demons. These demons weren't easy to kill, there wasn't any one who had the power to kill them. The gods came to power, and bore children with mortals. These demigods, in their varying different forms, Hindu, Norse, Greek, Roman, etc., fought the monsters of their people. Put together, these monsters are considered demons by the Nephilim. During the Crusades, a group of knights summoned the angel Raziel. Raziel bestowed upon them the 3 Mortal Instruments: the Cup, the Sword, and the Mirror. The Sword gave the holder an impossible urge to tell the truth, or else it would cause great pain. The Cup, which Raziel poured his blood into, gave the drinker the abilities of a Nephilim, essentially making them Nephilim. The Mirror, in my opinion, is just a relic with no real use," Magnus said, pleased with the look of confusion upon the demigod's faces.

Jace, meanwhile, was busy explaining a battle map to Annabeth. Of course, the only two that understood the map other than her were Jason and Frank. Jason pointed out an obvious issue with the map being that it was a map of New York rather than a map of Brocelind Plain. Jace gave him a look that said _'Don't judge me. It might be the last thing you do.'_ He then continued explaining the map.

Leo, ever impatient, was talking to Alec and Isabelle about the silvery _adamas_, which seemed very useful. Unfortunately, it could only be wielded by a Nephilim. In turn, Leo showed the Lightwoods how to shape Celestial Bronze weaponry. The Lightwoods were impressed when they saw his Celestial Bronze hammer slice the _adamas_ neatly into two pieces.

Clary hung back, talking to Reyna and Thalia. She was telling them a bit about how she had fallen for Jace in the first place, something Thalia found amusing. Reyna, however, found it endearing that the two were so perfect together. This saddened her because she never found love.

The door to Magnus' apartment crashed open that moment, revealing a black-haired child with glasses and a T-Shirt emblazoned with the _Fullmetal Alchemist_ logo. Behind him came Simon, clutching a similar shirt in his hand. The young boy grinned as he saw Isabelle and Alec.

"MAX!" Isabelle and Alec yelled in delight. Soon, Max was being crushed by hugs from his two older siblings. "Can't… breathe… crushing… Alec," Max croaked. Alec let go of his brother and looked at Simon. "Well?"

Simon looked down at his feet. "I found something that might sound crazy…".


End file.
